selftests/bpf: Add uprobe_multi usdt test code

Adding code in uprobe_multi test binary that defines 50k usdts
and will serve as attach point for uprobe_multi usdt bench test
in following patch.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-25-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add uprobe_multi test program

Adding uprobe_multi test program that defines 50k uprobe_multi_func_*
functions and will serve as attach point for uprobe_multi bench test
in following patch.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-23-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>